Trench structures are widely used in the semiconductor arts. Trenches may be used for a variety of purposes although they are most commonly used for isolation. Trench isolation may be utilized to isolate devices, CMOS portions and the like.
Polysilicon buffered LOCOS (PBL) is commonly used to form field oxidation regions in semiconductor structures. Typically, a substrate is provided and an oxide layer is formed thereon. A polysilicon layer is formed on the oxide layer and a nitride layer is formed on the polysilicon layer. An opening is formed through the nitride layer that exposes the polysilicon layer. Once the polysilicon layer is exposed, it and the substrate are thermally oxidized to form field oxidation regions. The PBL thermal oxidation step results in birds beak encroachment as is well known in the art. This encroachment is undesirable and does not allow PBL to be a viable isolation scheme in very small geometry structures.
PBL is commonly employed with nitride spacers. Following the formation of the opening, nitride spacers are formed on the sidewalls thereof. This allows the opening to have a width that is less than standard lithography resolution limits will allow. PBL utilized with nitride spacers is referred to frame masked polysilicon buffered LOCOS (FMPBL).
The basic integration of FMBPL and trench technology to form field oxidation regions above isolation trenches would appear to have many desirable features. However, this integration also renders substantial processing problems. Specifically, following the formation of a trench including a liner and polysilicon trench fill, the liner is etched back below the surface of the polysilicon trench fill. This etch back causes a recessed surface or divots in the liner. When nitride spacers are formed, nitride residue remains in the divots. During later processing steps, specifically when an oxide cap is grown over the trench, no oxide grows over the nitride in the divots. Therefore, when polysilicon is subsequently deposited and patterned over the trench, polysilicon stringers are formed in the divots. These polysilicon stringers are detrimental to semiconductor processing, final device performance and yield.
Accordingly, it would be highly desirable to have a method of fabricating a trench structure that would allow the desired features of FMPBL and trench technology without having the common problems associated with the integration of these technologies.